1. Field of the Invention
The present invention concerns an improvement in an iron making method and a steel making method for producing metallic iron by heat-reducing iron oxides (such as iron ores) together with a carbonaceous reducing agent (such as carbon material). More particularly, the present invention relates to an improved iron making method and a steel making method, wherein molten iron is produced by heat-reducing iron oxide-containing shaped products incorporating a carbonaceous reducing agent (pellets or briquettes) in a solid state, and further reducing and melting them. These methods are capable of improving heat efficiency in a series of steps from heat-reduction to reduction melting, and are capable of efficiently conducting separation of gangue components.
2. Description of the Prior Art
As a direct reduction process for producing reduced iron by reduction of iron oxides such as iron ores or iron oxide pellets with a carbon material or a reducing gas, a shaft furnace method typically represented by a MIDREX process has been known. In the direct iron making process of this type, reduced iron is obtained by a process of blowing a reducing gas produced from a natural gas or the like through a tuyere in a lower portion of a shaft furnace, and reducing iron oxide by utilizing the reducing force of the reducing gas. Further, a reduced iron production process of using a carbon material such as coal as a reducing agent instead of a natural gas has been noted in recent years and, specifically, a so-called SL/RN method of heat reducing sintered pellets of iron ores together with fine coal in a rotary kiln already has been put to practical use.
Further, as another iron making method, U.S. Pat. No. 3,443,931 discloses a process for producing reduced iron, which comprises mixing a carbon material and iron oxide fine into lumps, and heat-reducing them on a rotary hearth. In this process, the fine ores and the fine coal are mixed into lumps and then heat-reduced under a high temperature atmosphere.
Reduced iron produced by the above-mentioned method is utilized as an iron source by inserting as it is or after being formed into a briquette configuration at an atmospheric temperature to an electric arc furnace. Since the reduced iron contains less impurity metal components such as tramp elements, the reduced iron has been noted as a diluting material for tramp elements contained in the scraps in recent years in which recycling for iron scraps has become more and more active.
However, since slag components such as SiO.sub.2, Al.sub.2 O.sub.3 and CaO contained as gangue components in iron oxides (iron ore, etc.), carbon materials (coal or the like) intrude, and the iron quality of products (purity as the metallic iron) is lowered. In practical use, although the slag components are separated and removed in the succeeding smelting step, increase in the amount of the slags lowers the yield of the smelted molten iron, as well as gives marked undesired effects on the operation cost of the electric arc furnace.
Reduced iron with high iron content and with less slag content has been demanded. However, for satisfying such a demand by the existent production process for reduced iron described above, iron ores with high iron content have to be used for the raw material for producing reduced iron, which greatly narrows the range of selection for the iron making materials which can be used practically.
Further, the prior art method described above finally intends to obtain a reduced solid product as an intermediate product, and requires steps such as briquetting, cooling, transportation and storage until the delivery of the product to the reduction melting step as the succeeding step upon practical use, during which a large energy loss is caused, or additional facility or energy is required for briquetting.
On the other hand, as a method of obtaining a preliminary reduced iron by direct reduction of iron oxides, a smelting reduction method such as a DIOS method has also been known. This method comprises preliminarily reducing iron oxides by 30-50% as pre-reduction ratio, then reducing them to metallic iron by direct reducing reaction with solid carbon and/or carbon monoxide in an iron bath and slag and then melting the same. However, since a recycle system of producing a reducing gas required for the preliminary reduction step in a smelting reduction vessel and introducing it into the preliminary reduction furnace is constituted in this method, it is troublesome and extremely difficult to attain a balance of the process. In addition, since liquid iron oxides (FeO) and refractories are brought into direct contact with each other in a molten state, a problem of large corrosion of the refractories has been pointed out.
Further, Japanese Patent Publication Hei 3-60883 discloses other methods of mixing fine ores and carbon material, shaping them into agglomerates, preliminarily reducing them by a rotary hearth type heating furnace, charging the thus obtained preliminarily reduced products without cooling into a melting furnace, melting them, preceding reduction with addition of the carbon material and further blowing oxygen to conduct smelting. Since the preliminary reduction products are sent without cooling to the melting furnace and put to reduction and smelting in this method, it is considered that this method gives less heat energy loss, enables continuous operation and is effective also in view of productivity.
In this iron making method, oxygen (or air) is blown together with a great amount of carbon material into the melting furnace for heating and smelting. Then, since gangue components in the iron ores and the carbon material are contained in a great amount as described above in the preliminary reduction products sent to the melting furnace, a great amount of slag are exposed to violent stirring of the molten iron in the melting furnace. Since a great amount of iron oxides (FeO) is intruded in the slag, this results in a severe practical problem of remarkably causing erosion of lined refractories, so that it is difficult to make the method practical in an industrial scale.
Anyway, in order to ensure a reducing gas having a sufficient reduction potential required in the preliminary reduction furnace at the upper stream in the melting furnace, since it is necessary to supplement a great amount of oxygen and carbon material (several hundreds kg/tmi (mi: molten iron to be manufactured)) into the melting furnace burning them, the thermal load on the melting furnace is extremely large and the lined refractories undergo severe erosion by violent stirring of the molten iron and the slag. Further, for stably supplying the reducing gas at appropriate composition and amount required in the preliminary reduction furnace, it is extremely troublesome for attaining the balance over the entire facility, and a high level control system is required.